Investigation of effects equation of state and differential diffusion on fully developed stratified turbulent channel flow*

Density stratification in turbulent flows observed in flow systems such as sea, lakes, and oceans, usually depends upon two scalars, namely, temperature and salinity. The density field is expressed in terms of these scalars through an equation of state (EoS).  In addition, the molecular diffusion of temperature and salinity differs by an order of magnitude leading to differential diffusion phenomenon. In this study, the effects of linear/nonlinear EoS and differential diffusion on the instantaneous flow features and turbulent statistics in a fully developed stably stratified turbulent channel flow are examined using direct numerical simulations. The flow in the channel is simulated at a fixed friction Reynolds number of 180 and increasing level of stratification obtained by varying the friction Richardson number. The role of a mild differential diffusion is characterized by specifying two values of Schmidt number, namely, 1 and 4 for the salinity field and specifying Prandtl number to 1 for the temperature field. The results from the cases employing linear/nonlinear EoS and differential diffusion are compared with the corresponding reference simulations without considering salinity and EoS, where the transport equation for density is explicitly solved.